Fix diveSubfolder string termination

Firmware/cardreader.cpp

@@ -340,9 +340,9 @@ void CardReader::diveSubfolder (const char *fileName, SdFile& dir)
             {
                 const size_t maxLen = 12;
                 char subdirname[maxLen+1];
-                subdirname[maxLen] = 0;
                 const size_t len = ((static_cast<size_t>(dirname_end-dirname_start))>maxLen) ? maxLen : (dirname_end-dirname_start);
                 strncpy(subdirname, dirname_start, len);
+                subdirname[len] = 0;
                 SERIAL_ECHOLN(subdirname);
                 if (!dir.open(curDir, subdirname, O_READ))
                 {